brush maintenance

[Jeff Howard]

Hi guys,I'm new to this great forum!
I run with the Classic Thunder Club,and we use Astro 25 turn reversed. I've read about car guys drilling their brushes and sanding down the tips on the ends to reduce sparking. Does any one do/recomend this. Seems to me you just reduce comm contact. Any help would be great!:)
Jeff
Classic Thunder "Go fast,turn LEFT!"

[Jeff Wohlt]

Drilling a hole in the middle of the brush seems to be what most are doing these days. Jay Turner can tell you more but even guys running pletts do this.

Some factory speed brushes already have this done when you buy them. There is also a tool out to do it by somebody or campany I have seen it but can't remeber who made it.

Why you ask?.....lower resistance could be one part of it. Surface area on the comm.

[FrankW]

From what I read a while ago, the electricy travels through the outer sides of the brush. By drilling a hole in the center of the brush, you reduce the contact patch of the brush which will allow higher rpms, but shouldn't impact electricy flow much, if at all. This is what I've read from the R/C Car action forum.

-Frank

[Patrick M]

The hole in the brush also can be used as a "well" to hold comm drops. One theory states that electrons flow more readily from the brush edges, so if you create more edges by drilling a hole or serrating the brush face, the brush will have less friction and also provide better current flow.

Astroflights seem to run better with a slight chamfer on both the leading and trailing edge of the brush, this reduces timing overlap slightly and gives better efficiency.

[vicman]

There are many theories behind the "HOLES". The truth is, from experience, that they DO help but, not for the reasons you think. The biggest advantage is heat build-up. The hole in the center allows for the heat to be directed to the outer portions of the brush where cooling is better. It also cuts down on the contact or friction area of the brush. Slotting brushes, grooves, and side cuts are also big helpers. You can even change the timing of a roar stock motor by back cutting brushes. And its legal some times! The last thing is you can double the life of the comm by offset cutting the brushes. NOTE: brushes dont last very long doing these things though.

Jay could offer more info than myself, but these are some things have been done in the quest for speed.

vicman

[Jay Turner]

Be careful when trying to transfer what works in 05 car motors to the AstroFlight Cobalt motors. Astro brushes are a LOT different than 05 brushes in design and composition. They don't like most comm drops at all, they operate at lower face pressures, and they do not like to be offset cut. Actually, offset cutting brushes is pretty obsolete today, and was really only useful in low-power car applications anyway. Offsetting dramatically reduces power transfer to/from the comm in high-power applications and increases brush wear unless the springs are weakened.

Weakening the springs usually reduces overall power on 05s....except that the Astros operate best with relatively weak springs. Reducing the area of the brush face ( holes, offsetting, excessive camfering ) increases the pressure that the brush exerts on the comm - not always a good thing. Simply copying what works on an 05 can mess up an Astro....

What works? IME: fresh, completely broken-in brushes with a very slight camfer on both leading and trailing edges, a freshly skimmed comm and optimized timing. The 4-turn motors are not as robust as the earlier 5 and 6 turn versions, so be sure to let the motor cool off completely between runs if you're drawing a lot of amps looking for maximum power from the motor. If you don't then expect to eventually see a flowered comm - not a pretty sight! :(

[vicman]

Jay,
Thanks for pointing that out. I was indeed ONLY considering car motors, no exotics. I should have been more clear on this point, but was kinda fogged by the double ended question. Not a good excuse, should have paid more attention to the question. OOOps!

NOTE: brushes dont last very long doing these things though.

Now this has been twice said, hopefully it will sink in.

vicman

[JimClark]

Jay can you elaborate on the effects of comm drops on the Astro 25 brushes?
Thanks
Jim Clark



Originally posted by Jay Turner
They don't like most comm drops at all :(

[Jay Turner]

My experience ( and that of several others ) was that the rather soft, high silver content Astro 05 brushes softened when drops were used, causing a "gunk" to form which cloged the spaces between the comm plates. I haven't tried the newer synthetic Tribo drops - they may perform differently, but the 'standard' drops of the '90s didn't do the motors any favors....

[Patrick M]

To add to Jay's comments, the Astroflight comm does not hold up well to heat or excessive rev's. Keep the comm and brushes clean, and be careful in rough water situations where the prop is leaving the water on occasion. Over revving can easily result in that not so uncommon flower known as 'astrofliticus explodicus'.

[Spuds]

To add even more to Jay's comments, I've had bad luck with gunk buildup with 'standard' comm drops on both Astros and Mega's ... I have been using the Tribo drops for several months now nad have been quite happy, no gunk build up at least on my Mega's, my old Astro was sold before I changed over.

[Jeff Howard]

Thanks for the info guys, I think I'll leave the brushes alone. Just try to water cool them. They held up well with a half dozen or so races this last season,still look good. This 'astrofliticus explodicus' sounds real bad! More like a weed to me!LOL.

Jeff Howard--"Go fast,turn LEFT!"

[AndyKunz]

Originally posted by FrankW
From what I read a while ago, the electricy travels through the outer sides of the brush.

Frank,

I tried to debunk that myth here again just last week.

Simple logic:

If electricity travels on the skin of the wire, then resistance is linearly proportional to the reciprocal of the radius (because c=2PI x r).

If electricity travels down the whole wire, then resistance is proportional to the square of the radius (because A= PI x r^2).

Wire tables tell you the area of the wire "face" (the PI x r^2 one). Why would they do that if resistance were linear according to the diameter? Why are the tables I use daily for putting traces on a PCB referring to the weight and width of the trace (l x w, or surface area of the "face") in a squares table?

If you want the more complete version, do a search on RRR for "Renco Wire Table".

Andy

[NickPasch]

Hi Andy,

I think that lots of people have heard of the "skin effect", but most don't realize it is a phenomenon that is seen in the high mHz range. At 50-100 mHz it's impact is negligable, at the essentially DC (<20,000 kHz)conditions of esc's and batteries it's a complete non-issue.

Now there is always silver... ;)


Nick